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.Dd December 26, 2023
.Dt REGCOMP 3C
.Os
.Sh NAME
.Nm regcomp ,
.Nm regexec ,
.Nm regerror ,
.Nm regfree
.Nd regular-expression library
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In regex.h
.Ft int
.Fo regcomp
.Fa "regex_t *restrict preg" "const char *restrict pattern" "int cflags"
.Fc
.Ft int
.Fo regexec
.Fa "const regex_t *restrict preg" "const char *restrict string"
.Fa "size_t nmatch" "regmatch_t pmatch[restrict]" "int eflags"
.Fc
.Ft size_t
.Fo regerror
.Fa "int errcode" "const regex_t *restrict preg"
.Fa "char *restrict errbuf" "size_t errbuf_size"
.Fc
.Ft void
.Fn regfree "regex_t *preg"
.Sh DESCRIPTION
These routines implement
.St -p1003.2
regular expressions; see
.Xr regex 7 .
The
.Fn regcomp
function compiles an RE written as a string into an internal form,
.Fn regexec
matches that internal form against a string and reports results,
.Fn regerror
transforms error codes from either into human-readable messages,
and
.Fn regfree
frees any dynamically-allocated storage used by the internal form
of an RE.
.Pp
The translation of an RE into the internal form contained in a
.Ft regex_t
is inherently locale-specific; changes to the locale in effect between
.Fn regcomp
and subsequent calls to
.Fn regexec
may result in unexpected or undefined behavior.
.Pp
The header
.In regex.h
declares two structure types,
.Ft regex_t
and
.Ft regmatch_t ,
the former for compiled internal forms and the latter for match reporting.
It also declares the four functions, a type
.Ft regoff_t ,
and a number of constants with names starting with
.Qq Dv REG_ .
.Ss Fn regcomp
The
.Fn regcomp
function compiles the regular expression contained in the
.Fa pattern
string, subject to the flags in
.Fa cflags ,
and places the results in the
.Ft regex_t
structure pointed to by
.Fa preg .
The
.Fa cflags
argument is the bitwise OR of zero or more of the following flags:
.Bl -tag -width REG_EXTENDED
.It Dv REG_EXTENDED
Compile extended regular expressions
.Pq EREs ,
rather than the basic regular expressions
.Pq BREs
that are the default.
.It Dv REG_BASIC
This is a synonym for 0, provided as a counterpart to
.Dv REG_EXTENDED
to improve readability.
.It Dv REG_NOSPEC
Compile with recognition of all special characters turned off.
All characters are thus considered ordinary, so the RE is a literal string.
This is an extension, compatible with but not specified by
.St -p1003.2 ,
and should be used with caution in software intended to be portable to other
systems.
.Dv REG_EXTENDED
and
.Dv REG_NOSPEC
may not be used in the same call to
.Fn regcomp .
.It Dv REG_ICASE
Compile for matching that ignores upper/lower case distinctions.
See
.Xr regex 7 .
.It Dv REG_NOSUB
Compile for matching that need only report success or failure,
not what was matched.
.It Dv REG_NEWLINE
Compile for newline-sensitive matching.
By default, newline is a completely ordinary character with no special
meaning in either REs or strings.
With this flag,
.Qq [^
bracket expressions and
.Qq \&.
never match newline,
a
.Qq \&^
anchor matches the null string after any newline in the string in addition to
its normal function, and the
.Qq \&$
anchor matches the null string before any newline in the string in addition to
its normal function.
.It Dv REG_PEND
The regular expression ends, not at the first NUL, but just before the character
pointed to by the
.Va re_endp
member of the structure pointed to by
.Fa preg .
The
.Va re_endp
member is of type
.Vt "const char *" .
This flag permits inclusion of NULs in the RE; they are considered ordinary
characters.
This is an extension, compatible with but not specified by
.St -p1003.2 ,
and should be used with caution in software intended to be portable to other
systems.
.El
.Pp
When successful,
.Fn regcomp
returns 0 and fills in the structure pointed to by
.Fa preg .
One member of that structure
.Po other than
.Va re_endp
.Pc
is publicized:
.Va re_nsub ,
of type
.Ft size_t ,
contains the number of parenthesized subexpressions within the RE
.Po except that the value of this member is undefined if the
.Dv REG_NOSUB
flag was used
.Pc .
.Ss Fn regexec
The
.Fn regexec
function matches the compiled RE pointed to by
.Fa preg
against the
.Fa string ,
subject to the flags in
.Fa eflags ,
and reports results using
.Fa nmatch ,
.Fa pmatch ,
and the returned value.
The RE must have been compiled by a previous invocation of
.Fn regcomp .
The compiled form is not altered during execution of
.Fn regexec ,
so a single compiled RE can be used simultaneously by multiple threads.
The locale in effect at the time of
.Fn regexec
must be the same as the one in effect when the RE was compiled by
.Fn regcomp .
.Pp
By default, the NUL-terminated string pointed to by
.Fa string
is considered to be the text of an entire line, minus any terminating
newline.
The
.Fa eflags
argument is the bitwise OR of zero or more of the following flags:
.Bl -tag -width REG_STARTEND
.It Dv REG_NOTBOL
The first character of the string is treated as the continuation
of a line.
This means that the anchors
.Qq \&^ ,
.Qq [[:<:]] ,
and
.Qq \e<
do not match before it; but see
.Dv REG_STARTEND
below.
This does not affect the behavior of newlines under
.Dv REG_NEWLINE .
.It Dv REG_NOTEOL
The NUL terminating the string does not end a line, so the
.Qq \&$
anchor does not match before it.
This does not affect the behavior of newlines under
.Dv REG_NEWLINE .
.It Dv REG_STARTEND
The string is considered to start at
.Fa string No +
.Fa pmatch Ns [0]. Ns Fa rm_so
and to end before the byte located at
.Fa string No +
.Fa pmatch Ns [0]. Ns Fa rm_eo ,
regardless of the value of
.Fa nmatch .
See below for the definition of
.Fa pmatch
and
.Fa nmatch .
This is an extension, compatible with but not specified by
.St -p1003.2 ,
and should be used with caution in software intended to be portable to other
systems.
.Pp
Without
.Dv REG_NOTBOL ,
the position
.Fa rm_so
is considered the beginning of a line, such that
.Qq \&^
matches before it, and the beginning of a word if there is a word character at
this position, such that
.Qq [[:<:]]
and
.Qq \e<
match before it.
.Pp
With
.Dv REG_NOTBOL ,
the character at position
.Fa rm_so
is treated as the continuation of a line, and if
.Fa rm_so
is greater than 0, the preceding character is taken into consideration.
If the preceding character is a newline and the regular expression was compiled
with
.Dv REG_NEWLINE ,
.Qq ^
matches before the string; if the preceding character is not a word character
but the string starts with a word character,
.Qq [[:<:]]
and
.Qq \e<
match before the string.
.El
.Pp
See
.Xr regex 7
for a discussion of what is matched in situations where an RE or a portion
thereof could match any of several substrings of
.Fa string .
.Pp
If
.Dv REG_NOSUB
was specified in the compilation of the RE, or if
.Fa nmatch
is 0,
.Fn regexec
ignores the
.Fa pmatch
argument
.Po but see below for the case where
.Dv REG_STARTEND
is specified
.Pc .
Otherwise,
.Fa pmatch
points to an array of
.Fa nmatch
structures of type
.Ft regmatch_t .
Such a structure has at least the members
.Va rm_so
and
.Va rm_eo ,
both of type
.Ft regoff_t
.Po a signed arithmetic type at least as large as an
.Ft off_t
and a
.Ft ssize_t
.Pc ,
containing respectively the offset of the first character of a substring
and the offset of the first character after the end of the substring.
Offsets are measured from the beginning of the
.Fa string
argument given to
.Fn regexec .
An empty substring is denoted by equal offsets, both indicating the character
following the empty substring.
.Pp
The 0th member of the
.Fa pmatch
array is filled in to indicate what substring of
.Fa string
was matched by the entire RE.
Remaining members report what substring was matched by parenthesized
subexpressions within the RE; member
.Va i
reports subexpression
.Va i ,
with subexpressions counted
.Pq starting at 1
by the order of their opening parentheses in the RE, left to right.
Unused entries in the array
.Po corresponding either to subexpressions that did not participate in the match
at all, or to subexpressions that do not exist in the RE
.Po that is,
.Va i
>
.Fa preg Ns -> Ns Va re_nsub
.Pc
.Pc
have both
.Va rm_so
and
.Va rm_eo
set to -1.
If a subexpression participated in the match several times,
the reported substring is the last one it matched.
.Po Note, as an example in particular, that when the RE
.Qq (b*)+
matches
.Qq bbb ,
the parenthesized subexpression matches each of the three
.So Li b Sc Ns s
and then an infinite number of empty strings following the last
.Qq b ,
so the reported substring is one of the empties.
.Pc
.Pp
If
.Dv REG_STARTEND
is specified,
.Fa pmatch
must point to at least one
.Ft regmatch_t
.Po even if
.Fa nmatch
is 0 or
.Dv REG_NOSUB
was specified
.Pc ,
to hold the input offsets for
.Dv REG_STARTEND .
Use for output is still entirely controlled by
.Fa nmatch ;
if
.Fa nmatch
is 0 or
.Dv REG_NOSUB
was specified,
the value of
.Fa pmatch Ns [0]
will not be changed by a successful
.Fn regexec .
.Ss Fn regerror
The
.Fn regerror
function maps a non-zero
.Fa errcode
from either
.Fn regcomp
or
.Fn regexec
to a human-readable, printable message.
If
.Fa preg
is non-NULL, the error code should have arisen from use of the
.Ft regex_t
pointed to by
.Fa preg ,
and if the error code came from
.Fn regcomp ,
it should have been the result from the most recent
.Fn regcomp
using that
.Ft regex_t .
The
.Po
.Fn regerror
may be able to supply a more detailed message using information
from the
.Ft regex_t .
.Pc
The
.Fn regerror
function places the NUL-terminated message into the buffer pointed to by
.Fa errbuf ,
limiting the length
.Pq including the NUL
to at most
.Fa errbuf_size
bytes.
If the whole message will not fit, as much of it as will fit before the
terminating NUL is supplied.
In any case, the returned value is the size of buffer needed to hold the whole
message
.Pq including terminating NUL .
If
.Fa errbuf_size
is 0,
.Fa errbuf
is ignored but the return value is still correct.
.Pp
If the
.Fa errcode
given to
.Fn regerror
is first ORed with
.Dv REG_ITOA ,
the
.Qq message
that results is the printable name of the error code, e.g.
.Qq Dv REG_NOMATCH ,
rather than an explanation thereof.
If
.Fa errcode
is
.Dv REG_ATOI ,
then
.Fa preg
shall be non-NULL and the
.Va re_endp
member of the structure it points to must point to the printable name of an
error code; in this case, the result in
.Fa errbuf
is the decimal digits of the numeric value of the error code
.Pq 0 if the name is not recognized .
.Dv REG_ITOA
and
.Dv REG_ATOI
are intended primarily as debugging facilities; they are extensions,
compatible with but not specified by
.St -p1003.2 ,
and should be used with caution in software intended to be portable to other
systems.
.Ss Fn regfree
The
.Fn regfree
function frees any dynamically-allocated storage associated with the compiled RE
pointed to by
.Fa preg .
The remaining
.Ft regex_t
is no longer a valid compiled RE and the effect of supplying it to
.Fn regexec
or
.Fn regerror
is undefined.
.Sh IMPLEMENTATION NOTES
There are a number of decisions that
.St -p1003.2
leaves up to the implementor,
either by explicitly saying
.Qq undefined
or by virtue of them being forbidden by the RE grammar.
This implementation treats them as follows.
.Pp
There is no particular limit on the length of REs, except insofar as memory is
limited.
Memory usage is approximately linear in RE size, and largely insensitive
to RE complexity, except for bounded repetitions.
.Pp
A backslashed character other than one specifically given a magic meaning by
.St -p1003.2
.Pq such magic meanings occur only in BREs
is taken as an ordinary character.
.Pp
Any unmatched
.Qq \&[
is a
.Dv REG_EBRACK
error.
.Pp
Equivalence classes cannot begin or end bracket-expression ranges.
The endpoint of one range cannot begin another.
.Pp
.Dv RE_DUP_MAX ,
the limit on repetition counts in bounded repetitions, is 255.
.Pp
A repetition operator
.Po
.Qq \&? ,
.Qq \&* ,
.Qq \&+ ,
or bounds
.Pc
cannot follow another repetition operator.
A repetition operator cannot begin an expression or subexpression
or follow
.Qq \&^
or
.Qq \&| .
.Pp
.Qq \&|
cannot appear first or last in a (sub)expression or after another
.Qq \&| ,
i.e., an operand of
.Qq \&|
cannot be an empty subexpression.
An empty parenthesized subexpression,
.Qq () ,
is legal and matches an empty (sub)string.
An empty string is not a legal RE.
.Pp
A
.Qq \&{
followed by a digit is considered the beginning of bounds for a bounded
repetition, which must then follow the syntax for bounds.
A
.Qq \&{
.Em not
followed by a digit is considered an ordinary character.
.Pp
.Qq \&^
and
.Qq \&$
beginning and ending subexpressions in BREs are anchors, not ordinary
characters.
.Sh RETURN VALUES
On successful completion, the
.Fn regcomp
function returns 0.
Otherwise, it returns an integer value indicating an error as described in
.In regex.h ,
and the content of preg is undefined.
.Pp
On successful completion, the
.Fn regexec
function returns 0.
Otherwise it returns
.Dv REG_NOMATCH
to indicate no match, or
.Dv REG_ENOSYS
to indicate that the function is not supported.
.Pp
Upon successful completion, the
.Fn regerror
function returns the number of bytes needed to hold the entire generated string.
Otherwise, it returns 0 to indicate that the function is not implemented.
.Pp
The
.Fn regfree
function returns no value.
.Pp
The following constants are defined as error return values:
.Pp
.Bl -tag -width "REG_ECOLLATE" -compact
.It Dv REG_NOMATCH
The
.Fn regexec
function failed to match.
.It Dv REG_BADPAT
Invalid regular expression.
.It Dv REG_ECOLLATE
Invalid collating element referenced.
.It Dv REG_ECTYPE
Invalid character class type referenced.
.It Dv REG_EESCAPE
Trailing
.Qq \&\e
in pattern.
.It Dv REG_ESUBREG
Number in
.Qq \&\e Ns Em digit
invalid or in error.
.It Dv REG_EBRACK
.Qq []
imbalance.
.It Dv REG_ENOSYS
The function is not supported.
.It Dv REG_EPAREN
.Qq \e(\e)
or
.Qq ()
imbalance.
.It Dv REG_EBRACE
.Qq \e{\e}
imbalance.
.It Dv REG_BADBR
Content of
.Qq \e{\e}
invalid: not a number, number too large, more than two
numbers, first larger than second.
.It Dv REG_ERANGE
Invalid endpoint in range expression.
.It Dv REG_ESPACE
Out of memory.
.It Dv REG_BADRPT
.Qq \&? ,
.Qq *
or
.Qq +
not preceded by valid regular expression.
.El
.Sh USAGE
An application could use:
.Bd -literal -offset Ds
regerror(code, preg, (char *)NULL, (size_t)0)
.Ed
.Pp
to find out how big a buffer is needed for the generated string,
.Fn malloc
a buffer to hold the string, and then call
.Fn regerror
again to get the string
.Po see
.Xr malloc 3C
.Pc .
Alternately, it could allocate a fixed, static buffer that is big enough to hold
most strings, and then use
.Fn malloc
allocate a larger buffer if it finds that this is too small.
.Sh EXAMPLES
Matching string against the extended regular expression in pattern.
.Bd -literal -offset Ds
#include <regex.h>

/*
* Match string against the extended regular expression in
* pattern, treating errors as no match.
*
* return 1 for match, 0 for no match
*/
int
match(const char *string, char *pattern)
{
	int status;
	regex_t re;

	if (regcomp(&re, pattern, REG_EXTENDED\||\|REG_NOSUB) != 0) {
		return(0);      /* report error */
	}
	status = regexec(&re, string, (size_t) 0, NULL, 0);
	regfree(&re);
	if (status != 0) {
		return(0);      /* report error */
	}
	return(1);
}
.Ed
.Pp
The following demonstrates how the
.Dv REG_NOTBOL
flag could be used with
.Fn regexec
to find all substrings in a line that match a pattern supplied by a user.
.Pq For simplicity of the example, very little error checking is done.
.Bd -literal -offset Ds
(void) regcomp(&re, pattern, 0);
/* this call to regexec() finds the first match on the line */
error = regexec(&re, &buffer[0], 1, &pm, 0);
while (error == 0) {    /* while matches found */
	/* substring found between pm.rm_so and pm.rm_eo */
	/* This call to regexec() finds the next match */
	error = regexec(&re, buffer + pm.rm_eo, 1, &pm, REG_NOTBOL);
}
.Ed
.Sh ERRORS
No errors are defined.
.Sh CODE SET INDEPENDENCE
.Sy Enabled
.Sh INTERFACE STABILITY
.Sy Standard
.Sh MT-LEVEL
.Sy MT-Safe with exceptions
.Pp
The
.Fn regcomp
function can be used safely in a multithreaded application as long as
.Xr setlocale 3C
or
.Xr uselocale 3C
are not being called to change the locale.
.Sh SEE ALSO
.Xr attributes 7 ,
.Xr locale 7 ,
.Xr regex 7 ,
.Xr standards 7
.Pp
.St -p1003.2 ,
sections 2.8
.Pq Regular Expression Notation
and
B.5
.Pq C Binding for Regular Expression Matching .
